I'll start:
OK, well, I'm a sucker for silent film, and I love good three and four hour silent epics that bores the common plebian to tears. D.W. Griffith's INOLERANCE (1916) and Erich Von Stroheim's GREED (1924) are two masterpieces that absolutely blew me away upon first viewing. My only regret is that I don't usually have a spare three or four hours to watch these films more often. I also love silent comedy, so we'll add Charlie Chaplin's THE KID (1921) and Buster Keaton's THE GENERAL (1927) to my list. THE KID has a beautiful mix of comedy and drama, the best child performance I've ever seen from anyone by Jackie Coogan, and a scene with Chaplin reuniting with Coogan after a rooftop chase that makes me cry like a little girl everytime I see it.
THE GENERAL is one of those films that gets more amazing and even funnier the more I see it. There is so much detail that if you literally blink your eye, you can miss something.
I also love Universal Horror movies and am a big Bela Lugosi fan. I guess for the purpose of this list, I'll include THE RAVEN (1935). Bela's acting is more over the top in this film than anything else he's ever done, playing a sexually deprived retired doctor who like to torture his victims with devices inspired by Edgar Allan Poe. Karloff is pretty good here as well, but THE RAVEN is Lugosi's show. Artistically not the best Universal Horror (that distinction would belong to either BRIDE OF FRANKENSTEIN or THE BLACK CAT), THE RAVEN I still find loads of fun.
TWENTIETH CENTURY is a classic screwball comedy from 1934 with John Barrymore and Carole Lombard giving classic performances with an equally effective supporting cast. John Barrymore is espcially wonderful in this one, sending up his ham actor image perfectly, God bless him. I CLOSE THE IRON DOOR ON YOU!
THIS IS SPINAL TAP (1984) is a perfect parody of a topic that is near and dear to my heart - heavy metal music. Every scene is a classic and 99% f the time, I can think of bands who have been in similar situations. DUCK SOUP (1933) is the greatest Marx Brothers comedy that I want to include on my list and like THIS IS SPINAL TAP, every scene is a classic skit unto itself.
CITIZEN KANE (1941) also has to be included in my list. I know, I know, this is a safe movie to include because it's so universally praised, yet the hype is well deserved. A movie you can see 150 times and still find something new in. Every frame, literally, is a work of art into itself. As a film minor in college, I've written a few papers on this film and have gotten A's everytime. I think Orson Welles gave me an easy topic to write on, as you don't have to look hard to find something interesting to write about.
Finally, I'll include THE LADY EVE (1941). My favorite Preston Sturges comedy with a very sexy Barbara Stanwyck leading the way. She's a beautiful woman physically, but I feel Barabara Stanwyck had a certain attitude about her that made her sex appeal go beyond what her looks alone would convey. A very funny movie that is mandatory viewing for all fans of classic comedy.